1. Field of the Invention
The present invention relates generally to liquefied natural gas (LNG) terminals, and more particularly to LNG receiving terminals.
2. Background of the Invention
LNG is the liquid state of the same natural gas as used for gas-fired appliances in domestic households and industries, for pipeline sendout, and for electricity generation in gas-fired power plants. While natural gas in its gaseous state is used for domestic and commercial applications, when natural gas is transported from production locations to usage locations over long distances it is usually transported in a liquid state because LNG is about six hundred times smaller in volume than in its gaseous state. This significant reduction in volume makes LNG considerably less expensive than gaseous natural gas to transport over long distances. Hence, many LNG supply networks subject natural gas to liquefying at a production location, transporting between the production location and a usage location, and finally re-gasifying at the usage location prior to distribution to a consumer.
Different natural gas consumers, however, have different requirements for the LNG being re-gasified, such as varying calorific value and/or quality requirements. In order to satisfy different customer requirements, gas companies set strict requirements on the composition of the natural gas sent out of their LNG receiving terminals. These requirements vary from one LNG buyer to another, and often include Ethane (C2), Propane (C3) and heavier components content specifications that are lower than LNG production at existing LNG baseload plants. Exemplary pipeline specifications (Table 1) and LNG baseload plant outputs (Table 2) are provided below.
TABLE 1Exemplary Pipeline SpecificationsCalifornia AirResources BoardMexicon NaturalComponent, wt %MinimumCNG MaximumGas MaximumMethane (C1)88Ethane (C2)6Propane (C3+)33.6
TABLE 2Exemplary LNG Baseload OutputDasIsland,WhitnellRasComponentAbuBay,BintuluArun,Lumut,Botang,Laffan,wt %DhabiAustraliaMalaysiaIndonesiaBruneiIndonesiaQatarMethane87.1087.8091.2089.2089.4090.6089.60(C1)Ethane11.408.304.288.586.306.006.25(C2)Propane1.272.982.871.672.802.482.19(C3)Butane0.1410.8751.360.5111.300.821.07(C4)Pentane0.001—0.010.02—0.010.04(C5)
In many instances, LNG baseload plants cannot be efficiently modified so as to meet the varying specifications. This inflexibility is due, in part, to the configuration and equipment used in typical LNG baseload plants. Specifically, after an initial feed-gas treatment (e.g., acid-gas removal, dehydration, mercury removal, etc.), LNG baseload plants typically remove components from the LNG using a scrub column. As an example, benzene and C5 components may be removed from the LNG to prevent the LNG from freezing in a main cryogenic heat exchanger. Further, C2 components may be removed from the LNG to control the calorific value. Hence, many LNG baseload plants would have to modify the scrub column or alter its operation to satisfy the noted customer requirements.
The scrub columns at many baseload plants, however, cannot be effectively modified to satisfy customer requirements because doing so would reduce the operating pressure of feed gas entering the main cryogenic heat exhanger to unacceptable levels. Specifically, the feed-gas pressure for most baseload LNG plants is greater than 60 bara. If the plant must remove heavier hydrocarbon components to meet a typical North American market calorific value (e.g., about 1,070 btu/cu ft), the scrub column must operate at a pressure of about 40 bara based on the critical pressure of the feed gas. The critical pressure is “critical” because the separation process becomes difficult and very inefficient near the critical pressure while the refrigerant efficiency depends on the operating pressure of feed gas entering the main cryogenic heat exchanger. A lower calorific value, therefore, would require recompression of feed gas from the scrub column to the main cryogenic heat exchanger, which is significantly more expensive. As such, a need exists for a method and apparatus for reducing the amount of various components (e.g., C2 and/or C3+) without raising costs to prohibitive levels.
Other problems with the prior art not described above can also be overcome using the teachings of the present invention, as would be readily apparent to one of ordinary skill in the art after reading this disclosure.